Optical method for light diffraction, corresponding optical system and device

ABSTRACT

The invention concerns a light diffraction optical method covering a wide wavelengths range, a optical system and a optical measuring device corresponding.  
     According to the optical method, at least one incident light beam ( 21 ) is sent to a surface of an optical system ( 1 ), according to a angle of incidence (α) relative to the normal line ( 20 ) to the surface, the optical system comprising a Bragg reflector ( 2 ) and a grating ( 3 ) engraved on the Bragg reflector ( 2 ) on the surface. At least one returned beam ( 22 ) is sent by the optical system according to an angle of diffraction (β) relative to the normal line after diffraction.  
     The wavelengths and the angle of incidence of at least one of the incident beams are such that this incident beam is diffracted by the Bragg reflector and/or by the grating.

[0001] This invention concerns a light diffraction optical method, aswell as corresponding optical system and device.

[0002] The range of application of the invention also relates towavelengths measurements taken on monochromatic luminous beams as wellas the dispersion of polychromatic beams, for example in amonochromator.

[0003] The use of diffraction grating for the diffraction of light iswell known. Thus, in the vacuum ultraviolet range (wavelengths greaterthan 0.6 nm) or VUV, an Incident beam is sent conventionally under agrazing incidence to a diffraction grating and a returned beam iscollected for a given order of diffraction, according to an angle ofdiffraction complying with the laws of the gratings (diffraction ofFresnel). The gratings used, for example for a synchrotron radiationVUV, are typically engraved in materials such as silicon or SiC CVD, byionic engraving or rolling.

[0004] The wavelength range that may be covered by such gratings issomehow limited Internally. Indeed, at constant angle of incidence, thereflectivity decreases as a function of the wavelength. Satisfactoryefficiency of diffraction can not thus be easily obtained below 0.6 nm.

[0005] For diffractions at lower wavelengths, other systems, such asdiffracting crystals, are implemented. In such a crystal, having a givendistance between reticular planes, incident beams are diffractedaccording to the Bragg law. The useful wavelength range is delineatedabove by the distance between the reticular planes, and it is moreoverlimited in that the angular range is generally comprised between 5° and85° for practical reasons. For instance, for an oriented siliconmonocrystal (111), for which the double distance between reticularplanes is equal to 0.627 nm, the wavelength range varies between 0.055nm and 0.625 nm.

[0006] One may also perform Bragg reflectors by stacking of thin layerson a substrate. They also diffract the light according to the Bragg law.Below, by Bragg reflector is meant a crystal or a stack of layers on asubstrate.

[0007] Thus, to cover a wavelength range comprised for example between0.1 nm and 2 nm, it is necessary to use two devices and twoimplementations completely distinct, one of them enabling to work in theVUV range (diffraction grating) and the other in the X-ray range(diffraction crystal). Still, it appears useful in some cases to work ina range covering both these ranges, for example for a monochromatorcollecting a synchrotron radiation beam.

[0008] The invention concerns a light diffraction optical methodimplementing a diffraction grating, enabling to cover an extendedspectral range, extending for example from the vacuum ultraviolet tohard X-rays. More accurately, the optical measuring method of theinvention enables to take into account wavelengths ranging from 0.1 nmto 20 nm or more, by means of a single device, easily and economically.

[0009] The invention also concerns an optical system and an opticalmeasuring device having the advantages aforementioned.

[0010] To this end, the invention relates to an optical measuring methodwherein:

[0011] at least one incident light beam having at least one wavelengthis sent onto a surface of an optical system having a normal axis,according to a is direction forming an angle of incidence relative tosaid normal axis, such optical system comprising a Bragg reflector and agrating engraved at the surface of the Bragg reflector, and

[0012] at least one returned beam is collected by the optical systemaccording to a diffraction direction forming an angle of diffractionwith respect to the normal axis, after diffraction of the incident beamsby the optical system.

[0013] According to the invention, the wavelengths and the angle ofincidence of at least one of the incident beams are such that thisincident beam is diffracted by the Bragg reflector and/or by thegrating.

[0014] Thus, the Bragg reflector whereon is engraved the grating, isused directly for the diffraction at low wavelengths.

[0015] The method of the invention thus enables to provide adissociation of the Fresnel diffraction (on the grating) and of theBragg diffraction (in the Bragg reflector), thereby providing doubleoperation in the VUV range (grating) and in the range of the X-rays(Bragg reflector).

[0016] The method of the invention thus enables to simplify considerablythe measuring protocol and to provide smaller equipment.

[0017] Advantageously, for this incident beam diffracted by the Braggreflector:

[0018] the Bragg reflector is a crystal,

[0019] the angle of incidence is comprised between 5° and 80° and/or

[0020] the wavelengths are ranged between 0.1 nm and 0.7 nm.

[0021] In a preferred embodiment, the crystal is formed of an siliconmonocrystal (111) and the grating is directly engraved on this crystal.Such a substrate proves particularly suitable to fulfil the doublefunction of crystal diffraction and of grating diffraction.

[0022] The grating is advantageously covered with a metallic layer. Theefficiency of the grating is thereby increased.

[0023] Preferably, the wavelengths and the angle of incidence of atleast another of the incident beams are such that this incident beam isdiffracted by the grating.

[0024] The double function of the optical system is thereby provided:diffraction by the crystal and diffraction by the gratingAdvantageously, for this other incident beam diffracted by the grating:

[0025] the angle of incidence is at least equal to 70° and/or

[0026] the wavelengths of the other incident beam are comprised between0.6 nm and 150 nm.

[0027] In a first implementation of the method, the incident beams arepolychromatic luminous beams. Then, advantageously, the method isimplemented in a monochromator, the optical system serving as adispersive element and followed by a selection slit.

[0028] In a second implementation of the method, the incident beams aremonochromatic luminous beams. The optical system may thus be used asprimary calibration means for measuring wavelengths.

[0029] The invention also concerns an optical system comprising a Braggreflector.

[0030] According to the invention, it comprises a diffraction gratingengraved on the Bragg reflector.

[0031] Preferably, the Bragg reflector is a crystal and the crystal iscomposed of a silicon monocrystal.

[0032] This optical system enables to implement the method of theinvention.

[0033] The invention also relates to an optical measuring devicecomprising:

[0034] an optical system according to the invention,

[0035] means for irradiating the surface of the optical system by meansof at least one incident beam of light,

[0036] means for collecting at least one returned beam by the opticalsystem after diffraction of the incident beams by the optical system,and

[0037] rotary means relative of the optical system with respect to theincident beams.

[0038] The invention also applies to the use of the method or of thedevice according to the invention for primary calibration for measuringwavelengths (monochromatic luminous beam) or for dispersion in amonochromator or spectrograph (polychromatic luminous beam).

[0039] This invention will be understood better and illustrated by meansof the following embodiments, without limitation thereto, with referenceto the appended drawings whereon:

[0040]FIG. 1 is a schematic diagram illustrating an optical system usedin the method according to the invention (the scales are not respectedfor better visibility);

[0041]FIG. 2 shows a profile, measured with a scanning tunnelingmicroscope (STM) of a modulation recorded on an optical system used toimplement an optical measuring method according to the invention;

[0042]FIG. 3 is a principle diagram of embodiment of the opticalmeasuring method according to the invention in diffraction crystal mode;

[0043]FIG. 4 is a principle diagram of the embodiment of the opticalmeasuring method according to the invention in a mode diffractiongrating;

[0044]FIG. 5 represents for the optical system of FIG. 2 and indiffraction crystal mode, the reflectivity as a function of thedifference of the angle of incidence to the angle of Bragg for awavelength equal to 0.154 nm;

[0045]FIG. 6 represents for the optical system of FIG. 2 and indiffraction grating mode, the efficiency as a function of the incidenceangle for the orders 1 and −1, for a wavelength equal to 1.33 nm;

[0046]FIG. 7 represents for the optical system of FIG. 2 and indiffraction grating mode, the efficiency as a function of the angle ofincidence for the orders 1 and −1, for a wavelength equal to 1.55 nm;and

[0047]FIG. 8 shows an optical measuring device used to implement anoptical measuring method according to the invention.

[0048] An optical system 1 (FIG. 1) comprises a Bragg reflector 2 and agrating 3 engraved on the substrate of the Bragg reflector 2 at asurface 8 of the optical system 1. The grating 3 is covered with ametallic layer 4, for example composed of a layer of 10 nm of gold.

[0049] The Bragg reflector 2 is advantageously composed of a siliconmonocrystal (111). It is super-polished, with a slope error of a fewtens of arc s conds and a roughness of a few Å. This polishing enablesthe operation of the optical system 1 in grazing reflection, for use invacuum ultraviolet implementing a diffraction by the grating 3.

[0050] In an alternative embodiment, the Bragg reflector 2 is a stack oflayers. It may be itself placed on a substrate.

[0051] The diffraction grating 3 is for example inscribed by holographicrecording and ionic machining. It comprises lines 5 (FIG. 2) whereof thedepth is for example smaller than 10 nm, which makes it a very littlemodulated grating. The profile of the grating 3 can be obtained byscanning tunneling microscopy, in height (depth of engraving), width andlength (respectively axes 11, 12 and 13, in nm). The density N of linesof the grating 3 per mm is for example equal to 1200.

[0052] For the lines 5, different shapes (sinusoidal, triangular, andsquare) and different density laws (constant or variable) may be used.

[0053] The optical system 1 is used to cover a spectral range from thevacuum ultraviolet to hard X-rays. Thus, according to a first embodiment(FIG. 3), an incident beam 21 is sent, having a wavelength smaller than0.6 nm on the surface 8. The optical system 1 having a normal axis 20 tothe surface 8, the incident beam 21 forms relative to this normal axis,an angle α and with respect to the reticular planes 6 of the Braggreflector 2 (i.e., in such case, relative to the surface 8), an angle θ.The angle α ranges preferably between 5° and 80°.

[0054] The optical system 1 then behaves like a conventional diffractioncrystal, the beams diffracted 22 by the Bragg reflector 2 forming withthe normal axis 20, an angle β equal to the angle α (FIG. 3). Thissystem 1 may also be used as a wavelength calibration or as adiffracting element of an X-ray monochromator.

[0055] The absence of perturbations of the Bragg diffraction in theBragg reflector 2 by the grating 3 may be explained in that the depth ofthe lines 5 of the grating is sufficiently small relative to the depthof penetration of the incident beam in the Bragg reflector 2, when suchincident beam has wavelengths which are sufficiently small (inparticular X-rays).

[0056] The spectral range covered is given by the Bragg law:

λ=2d sin θ

[0057] d designating the distance between the reticular planes.Consequently, for the Bragg reflector 2 of the example (siliconmonocrystal (111), the double 2d of the distance is equal to 0.627 nm.

[0058] Thus, the angle α being comprised between 50 and 80°, the usefulwavelengths range extends approximately from 0.1 nm to 0.625 nm.

[0059] Good results can also be obtained while using an oriented siliconmonocrystal (311).

[0060] According to a second embodiment, the optical system 1 is causedto operate as a diffraction grating in the vacuum ultraviolet range.Thus, (FIG. 4) an incident beam 25 is sent at a wavelength greater than0.6 nm. This incident beam 25 forms with the normal axis 20, an angle αenabling to provide high efficiency in the order of diffraction used,advantageously greater than or equal to 70°, so that the incidentradiation is quasi a grazing one. High efficiency is thereby maintained.The incident beam 25 then interacts with the diffraction grating 3 andgenerates diffracted beams 26 forming angles θ with the normal axis 20,such angles of diffraction β depending on the order of diffractionconsidered (the diffracted beam 26 represented on FIG. 4 corresponds forexample to the order −1).

[0061] The correct behaviour of the optical system 1 has been checkedfor both operating modes, respectively in Bragg diffraction and inFresnel diffraction. For the tests performed, the density of lines 5 bymm is equal to 1200 and the depth of the lines 5 is equal to 7.2 nm.

[0062] Thus, the response of the optical system 1 has been tested for afixed wavelength (0.154 nm) as a function of the incidence angle on anX-ray tube with a goniometer θ-2θ. In such an arrangement, when theangle of incidence varies by Δθ, the detector is rotated by 2Δθ, inorder to comply with the law of Bragg. On FIG. 5, as a function of thedifference of the incidence angle to the Bragg angle (given by the lawof Bragg, axis 14, in arc-seconds), the reflectivity is carried forwardfor the wavelength of 0.154 nm. It can be noted that the curve 31obtained has a width at half the maximum (FWHM) smaller than 20arc-seconds, whereas the result is equivalent to that obtained withconventional silicon crystals. This validates the use of the opticalsystem 1 in the X-ray range.

[0063] In order to test the operating mode in diffraction grating, twomonochromatic beams 25 have been sent in succession at two distinctwavelengths. For each of them, the efficiency of the grating 3 wasmeasured in the order −1 and +1 as a function of the angle of incidenceα. Moreover, k designating the order of diffraction, λ representing thewavelength and N being the number of lines 5 per mm of the grating 3,the detector was placed at such at an angle that the law of the gratingsis complied with:

sin α+sin β=kNλ.

[0064] For the wavelength of 1.33 nm (FIG. 6), the efficiency isrepresented (i.e. the ratio of the intensity of the flux of thediffracted beam 26 to the intensity of the flux of the incident beam 25,axis 17) as a function of the angle of incidence (axis 16, in degrees).For the orders −1 and 1, respectively sets of points 41 and 43 areobtained. They are compared respectively to theoretical curves 42 and 44calculated on the basis of the parameters of the grating 3, derived fromthe measurements realised with the scanning tunneling microscope.Similarly, for a wavelength of 1.55 nm (FIG. 7), respectively sets ofpoints 45 and 47 are plotted as well as the corresponding theoreticalcurves 46 and 48 for the diffraction orders −1 and +1.

[0065] It can be observed that the measurements obtained are very closeto the theoretical curves, which validates the operation of the opticalsystem 1 in diffraction grating mode, the latter diffracting theradiation with notable efficiency.

[0066] One may also use the optical system 1 in an optical measuringdevice (FIG. 8) comprising means for irradiating 51 the surface 8 of theoptical system 1 and means for collecting 52 beams returned by theoptical system 1 after diffraction of the incident beams. Such devicealso comprises rotary means 53 relative to the optical system withrespect to the incident beams, acting on the optical system 1 and/or onthe orientation of the incident beams 21 or 25, in order to producerelative rotation 54.

[0067] For instance, with the irradiating means 51, a monochromaticluminous beam is emitted and thanks to the rotary means 53, the opticalsystem 1 is oriented with respect to the incident beam in crystaldiffraction mode (angle of incidence a comprised between 5° and 80°) orin grating diffraction mode (angle of incidence α advantageously greaterthan or equal to 70°), according to the range to which belongs thewavelength of the beam processed. One then proceeds conventionally incrystal operation or in grating operation with the collection means 52.

[0068] In another example, the irradiating means 51 are the output of asynchrotron producing a polychromatic energy beam, and the opticalsystem 1 as well as the collection means 52 are used as a monochromator.The collection means 52 comprise notably a silt for selectingwavelengths. The device is th n implem nted in grating diffraction modeor in crystal diffraction mode, according to the wavelength(s) studied.

1. A light diffraction optical method wherein: at least one incidentlight beam (21, 25) having at least a wavelength is sent onto a surface(8) of an optical system (1) having a normal axis (20), according to adirection of Incidence forming an angle of incidence (α) relative tosaid normal axis (20), said optical system (1) comprising a Braggreflector (2) and a grating (3) engraved on the Bragg reflector (2) atsaid surface (8), and at least one returned beam (22, 26) is collectedby the optical system (1) according to at least one diffractiondirection forming an angle of diffraction (β) with respect to the normalaxis (20), after diffraction of the incident beam (21, 25) by theoptical system (1), characterised in that said wavelengths and saidangle of incidence (α) of at least one of the incident beams (21) aresuch that said incident beam (21) is diffracted by the Bragg reflector(2) and/or by the grating.
 2. An optical method according to claim 1,characterised in that the Bragg reflector is a crystal.
 3. An opticalmethod according to one of the claims 1 or 2, characterised in that saidangle of incidence (α) is ranged between 5° and 80°.
 4. An opticalmethod according to any of the claims 1 to 3, characterised in that saidwavelengths are ranged between 0.1 nm and 0.7 nm.
 5. An optical methodaccording to any of the claims 2 to 4, characterised in that the Braggreflector (2) is composed of oriented silicon monocrystals (111).
 6. Anoptical method according to any of the previous claims, characterised inthat the grating (3) is covered with a metallic layer (4).
 7. An opticalmethod according to any of the previous claims, characterised in thatsaid wavelengths and said angle of incidence (α) of at least another ofthe incident beams (25) are such that said incident beam (25) isdiffracted by the grating (3).
 8. An optical method according to claim7, characterised in that said angle of incidence (α) of said otherincident beam (25) is at least equal to 70°.
 9. An optical methodaccording to one of the claims 7 or 8, characterised in that saidwavelengths of said other incident beam (25) are comprised between 0.6nm and 150 nm.
 10. An optical method according to any of the previousclaims, characterised in that said incident beams are polychromaticluminous beams.
 11. An optical method according to any of the claims 1to 9, charact rised in that said incident beams are monochromaticluminous beams.
 12. An optical system (1) comprising a Bragg reflector(2), characterised in that it comprises a diffraction grating (3)engraved on the Bragg reflector.
 13. An optical system according toclaim 12, characterised in that the Bragg reflector is a crystal.
 14. Anoptical system (1) according to claim 13, characterised in that theBragg reflector (2) is composed of a silicon monocrystal.
 15. An opticalmeasuring device comprising: an optical system (1) according to one ofthe claims 12 to 14, means for lighting (51) said surface (8) of theoptical system (1) by means of at least one incident light beam (21,25), means for collecting (52) at least one returned beam (22, 26) bythe optical system (1) after diffraction of said incident beams (21, 25)by the optical system (1), and rotary means (53) relative of the opticalsystem (1) with respect to said incident beams (21, 25).
 16. A use ofthe optical measuring method according to any of the claims 1 to 11 orof the device according to one of the claims 12 to 15, for a primarycalibration for measuring wavelengths or for dispersion in amonochromator.